It has been known that a transparent and heat-sealable film, suitable for use as a corrosion-inhibiting packaging material for metal products, can be obtained by admixing a thermoplastic resin such as a polyolefin with a compound popularly known as a volatile corrosion inhibitor and extruding the resultant mixture in the form of a film. The vapor of the compound inhibits corrosion at the surfaces of the metal products when the metal products are sealed within the films. For the formation of such films, there have been proposed a variety of methods. However, thermoplastic resins and the volatile corrosion inhibitors generally are so incompatible that when they are mixed, the corrosion inhibitors are poorly dispersed within the resins, i.e. dispersion of the corrosion inhibitors in the resins is not satisfactorily uniform. Some of the volatile corrosion inhibitors have melting points higher than the molding temperature of the thermoplastic resins to be used. When the volatile corrosion inhibitor has a higher melting point, the corrosion inhibitor does not melt when the thermoplastic resin is extruded and the result is a poor dispersion. To effect this deficiency the volatile corrosion inhibitor may be used in an increased amount to enable the finally produced film to manifest a sufficiently high corrosion-inhibiting property. However, the use of such large amounts of inhibitor results in a degradation of transparency and strength of the formed film. Moreover, because of the poor dispersion of the corrosion inhibitor as described above, the volatile corrosion inhibitor contained within the film quickly bleeds to the surface of the film in a short period of time and consequently, by dissipating the vapor too quickly, the film can not retain the corrosion-inhibiting effect for a long period of time and the heat-sealability and strength of the film is reduced.
In the specification of Japanese patent publication No. 4,295/1972, there is disclosed a method for the production of a corrosion-inhibiting plastic film, wherein a polyolefin is used in combination with a polar resin such as an ethylene-ethyl acrylate copolymer, an ethylene-acrylic acid copolymer or an ethylene-vinyl acetate copolymer to improve the compatibility and dispersibility of the volatile corrosion inhibitor with the polyolefin and consequently produce a corrosion-inhibiting plastic film which contains the volatile corrosion inhibitor in a high ratio, which is not subject to surface bleeding of the corrosion inhibitor and which provides a lasting corrosion-inhibiting effect. This method is effective to some extent in uniformly dispersing the volatile corrosion inhibitor. Since this method requires the polyolefin to be used in combination with a considerably large amount of the polar resin (of the order of 15 to 40 parts by weight per 100 parts by weight of polyethylene in accordance with working examples cited in the specification of Japanese patent publication No. 4,295/1972), the corrosion-inhibiting film produced by this method combines the properties of the polyethylene and the polar resin at a sacrifice of the excellent properties of polyethylene film in packaging applications. To be specific, the polar resin has higher flexibility and a lower softening point than polyethylene. When they are blended and extruded in the form of a film, the film suffers from various disadvantages such as inferior strength, heat resistance and antiblocking.